Bump node environment to 15.x

It updates node version to 15.x to be able to use
`String.prototype.replaceAll()`

.github/workflows/test.yaml

@@ -21,7 +21,7 @@ jobs:
         name: Setup node
         uses: actions/setup-node@v1
         with:
-          node-version: '14.x'
+          node-version: 15.x
       -
         name: Install dependencies
         run: npm ci